1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon separation. More specifically the invention relates to an improved process for the separation of para-xylene from a feed stream comprising para-xylene and at least one other C.sub.8 aromatic isomer which process employs a particular solid adsorbent and a particular desorbent material to effect the selective adsorption and desorption of para-xylene.
2. Description of the Prior Art
The prior art has recognized that type X or type Y zeolites containing selected cations at the exchangeable cationic sites can be used to separate para-xylene from a feed mixture containing para-xylene and at least one other C.sub.8 aromatic isomer.
In my U.S. Pat. Nos. 3,558,730 and 3,663,638 for instance, I recognized the particular suitability of a crystalline aluminosilicate adsorbent containing both barium and potassium at the exchangeable cationic sites for use in a para-xylene separation process. My U.S. Pat. No. 3,734,974 discloses the particular effectiveness of an adsorbent comprising type X or type Y zeolites containing barium cations at the exchangeable cationic sites and water within the zeolite when used in a process for separating para-xylene.
I have also previously discovered the suitability of particular materials for use as desorbent materials in C.sub.8 aromatic adsorptive separation processes. In my U.S. Pat. No. 3,558,732 I discovered an improved process for separating at least one C.sub.8 aromatic isomer from a feed containing a mixture of C.sub.8 aromatic hydrocarbons wherein the improvement was employing a desorbent material containing toluene. A desorbent material containing toluene is particularly well suited for use in a process which separates para-xylene from an extracted feed stream, that is, one containing essentially no C.sub.8 non-aromatics. With such a desorbent material and with extracted feed streams separation of para-xylene in both high purity (greater than about 99%, expressed as a percent of C.sub.8 aromatics present) and high yields (greater than about 98%) is common. In my U.S. Pat. No. 3,686,342 I discoverd an improved process for separating para-xylene from a feed containing a mixture of C.sub.8 aromatics wherein the improvement was employing a desorbent material containing paradiethylbenzene. While this desorbent material can be used with extracted feed streams, it is particularly useful in separating para-xylene from non-extracted feed streams, that is, those containing C.sub.8 non-aromatics in varying concentrations. When a toluene desorbent is used with a nonextracted feed stream C.sub.8 non-aromatics, such as napthenes, which have a boiling point close to that of toluene, make clean separation and recovery of the toluene desorbent material from the extract and raffinate output streams difficult if not impossible resulting in eventual contamination of the toluene desorbent. Thus the use of a desorbent material comprising para-diethylbenzene extends the separation process to a wider variety of feed stocks, such as non-extracted C.sub.8 aromatic fractions. Being able to use non-extracted C.sub.8 aromatic fractions as feed streams to the para-xylene separation process eliminates the need for the aromatic extraction processing step otherwise required in the preparation of a feed stream to be used in a process employing toluene as a desorbent material.
I have discovered, however, that when a desorbent material containing para-diethylbenzene is used with certain adsorbents, for example those comprising type X or type Y zeolites containing barium and potassium at the exchangeable cationic sites, that the selectivity of the adsorbent is higher for the desorbent material than it is for para-xylene. This results in the inability of the para-xylene separation process employing this adsorbent and a desorbent material containing paradiethylbenzene to obtain yields of high purity (greater than about 99% expressed as a percent of the C.sub.8 aromatics present) para-xylene of greater than about 95%. Thus although about the same para-xylene purities can be obtained as those obtainable from a separation process using toluene as a desorbent material, the para-xylene yields are not as high as those from the separation process using toluene.
I have further discovered that when an adsorbent comprising type X or type Y zeolite containing barium and strontium at the exchangeable cationic sites is used in the para-xylene separation process with a desorbent material containing para-diethylbenzene that the problem is eliminated. Thus the process of my invention makes separation of paraxylene in both high purity (greater than about 99%) and high yields (greater than about 95%) possible. Para-xylene is a valuable raw material used to make polyester fibers, polyester films and polyethylene terephthalate and polybutylene terephthalate resins.